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1. Field of the Invention
The present invention relates to orthopaedic prosthetic devices, and more particularly to an improved rotating platform, mobile knee prosthesis that incorporates anterior stabilization along with the ability to constrain the movement of the articular surface from rotation and translation, to rotation only.
2. General Background of the Invention
Femoral rollback is believed to improve range of motion and extensor mechanism leverage so as to improve efficiency and more accurately replicate natural kinematics. Conventional mobile bearing designs may Lack the desired effect of femoral rollback, particularly in the absence of the posterior cruciate ligament.
Posterior Stabilized (PS) fixed bearing designs provide femoral rollback by articulating a cam on the femoral component with a post on the tibial articular insert during flexion. However, PS fixed bearing designs do not have the advantages of mobile bearing designs with regards to enhanced range of motion, reduced rehabilitation time, improved patellofemoral alignment, increased contact area, and reduced bone-implant interface shear forces.
In fixed bearing designs, excessive wear of the PS post can occur during articulation with the femoral cam. Internal-external rotation of the femoral component reduces the PS post-femoral cam congruency which increases contact stresses. The increased contact stresses can lead to excessive polyethylene wear and component failure. Allowing the PS post to rotate within a fixed articular insert will maintain femoral cam-PS post congruency during internal/external rotation of the femoral component.
Further, in any type of posterior stabilized design (fixed bearing or mobile bearing), one of the most problematic failure modes of the polyethylene is the fracture of the central post of the insert. This failure can be attributed to xe2x80x9cnotchingxe2x80x9d the anterior side of the central post with the anterior most inner-condylar area of the femoral component. Thus, any mechanism to reduce the probability for impingement of the femoral component against the anterior side of the tibial central post in hyper-extension would reduce the probability for tibial insert post failure due to xe2x80x9cnotchingxe2x80x9d and ultimately breaking.
Previous rotating platform designs have incorporated rotating only, or rotation and translation through the use of different prostheses. An example of a prosthesis that rotates and translates is shown in British publication 2219942, entitled xe2x80x9cKnee Prosthesisxe2x80x9d. U.S. Pat. No. 5,906,643 provides a tibial baseplate with a post that protrudes through a meniscal component and articulates with a cam on a femoral component. The post is an integral part of the tibial baseplate.
U.S. Pat. No. 5,879,392 provides a tibial baseplate with a fixed post that extrudes through the stem of the tibial baseplate and through the bearing component and articulates with a recess within the femoral component.
EP 0 916 321 A2 provides a femoral component with transverse flanges on the medial and lateral surfaces of the posterior stabilized box that articulate with projections from the medial and lateral surfaces of the post.
WO 95/35484 provides a bearing component with a post that articulates with a recess within the femoral component. The bearing component is limited in rotational, anterior, and posterior movement with respect to the tibial component.
The following patents relate to other orthopedic prosthetic devices, many of the listed patents pertaining to a knee prosthesis:
The present invention has as an object a tibial prosthesis and mating articular insert with specially configured stabilization posts. The invention enables for the surgeon to convert a mobile bearing articular surface from a fixed to a rotating only or translating only. The prosthesis can also provide both rotation and translation simultaneously.
These conversions are accomplished with special locking members or plugs that connect to the tibial base special plate. The plugs can be secured to the base plate with a taper lock or a threaded connection for example.
A post on the proximal tibial base plate can be positioned with an offset with respect to an oval hole in the articular insert to provide anterior stabilization in the total knee prosthesis.
The prosthesis of the present invention will be used as part of a total knee surgery when the surgeon chooses to use a prosthesis that incorporates a particular, selected relative motion between tibial tray and tibial insert.
This present invention consists of a posterior stabilized xe2x80x9cPSxe2x80x9d post which is secured to the mobile bearing tibial baseplate allowing only rotational movement. The PS post captures a bearing component to the tibial baseplate through an elongated slot in the bearing component. The elongated slot in the bearing component allows it to translate anteriorly and posteriorly with respect to the posterior stabilized post. The bearing component may also rotate with respect to the tibial baseplate in conjunction with the PS post. The bearing component has two concave surfaces that are articulate with the convex surfaces of the femoral component, and that are roughly congruent with the convex surfaces of the femoral component at zero degrees of flexion or full extension. The PS post articulates with a recess or cam of the femoral component to provide femoral rollback.
In addition to the above described design, the PS post should allow for posterior translation, in addition to rotational movement. This posterior movement would allow the post to translate instead of impinging upon the inner-condylar notch area of the femoral component in hyper-extension.
The PS post has a flat distal surface that articulates with the tibial baseplate. A T-slot is located on the distal end and-articulates with a T-post on the tibial baseplate. A through hole in the PS post is located such that a rotation peg can capture the PS post to the tibial baseplate while the T-slot of the PS post is engaged with the T-post of the tibial baseplate. The rotation peg allows only rotational freedom of the PS post with respect to is the tibial baseplate. The PS post has a flange on the medial and lateral surfaces that capture the bearing component through a counterbore on the medial and lateral sides of an elongated slot of the bearing component. The elongated slot of the bearing component is larger than the PS post in the anterior-posterior direction such that the bearing component has limited translation with respect to the PS post. The bearing component may also rotate with respect to the tibial baseplate in conjunction with the PS post. The bearing component has two concave surfaces that are congruent to the convex surfaces of the femoral component. A cam mechanism on the femoral component can be a concave cylinder that can be congruent to the convex posterior surface of the PS post. The internal/external rotation of the PS post with the femoral component can maintain this congruency throughout the range of motion unlike designs with a fixed PS post.
The addition of the posterior translation can occur with an anterior to posterior xe2x80x9cA/Pxe2x80x9d slot instead of a hole as seen in the inferior view above. This slot would allow for posterior translation of the post relative to the insert/baseplate.
The xe2x80x9cPSxe2x80x9d post may engage the tibial baseplate through a pin means or through a boss of a configuration other than a T-post. The xe2x80x9cPSxe2x80x9d post may secure the bearing component through the use of slots or other means of capture. The xe2x80x9cPSxe2x80x9d post may articulate with a closed recess within the femoral component rather than a cam mechanism.
With the fixed bearing design, the means of xe2x80x9cPSxe2x80x9d post capture may be with the use of a retaining ring or a cross pin. The PS post may not require capture with the fixed bearing articular insert.
The present invention also provides an improved knee prosthesis apparatus that includes a tibial prosthesis that is configured to be surgically implanted on a patient""s transversely cut proximal tibia and a femoral component. The femoral component articulate with a tibial insert having a proximal surface that engages the femoral component, the insert having a distal surface that fits against and articulates with the proximal surface of the tibial prosthesis.
A constraining mechanism joins the tibial insert to the tibial prosthesis in a selective fashion that enables a number of different possible relative motions between the insert and the tibial prosthesis, including anterior to posterior translation with rotation, or rotation only.
All or part of the constraining mechanism is separable from the tibial prosthesis, and selective removal of all or part of the constraining mechanism determines which of the said possible relative motions will take place.
The tibial prosthesis can have a fixator for holding the tibial prosthesis on a patient""s proximal tibia such as for example, a stem, spike, cement, etc.
The proximal surface of the insert can have one or more concavities for articulating with the femoral component.
The femoral component can include an intercondylar surface that is positioned to contact the post, enabling relative motion between the femoral component and the insert to be constrained.
An additional embodiment of the apparatus of the present invention provides a cam and bar arrangement that prevents xe2x80x9cspin outxe2x80x9d that can occur in mobile bearing tibia designs. In mobile bearing tibia designs, the tibial component is free to rotate relative to the femur. In one embodiment, the femoral component is provided with a transverse, specially shaped bar that cooperates with a tibia cam provided on the tibial insert. This improved arrangement forces a xe2x80x9croll backxe2x80x9d of the femoral component relative to the tibial component in flexion. The tibial component insert provides a pair of projecting portions, including a centrally located post having a posterior tibia cam surface and a posterior projecting portion that has a convex anterior surface. In between these two projecting portions, a slot is provided having a specially shaped bottom portion that xe2x80x9ccapturesxe2x80x9d the transverse bar of the femoral component, constraining the bar and thus the femoral component. This apparatus disallows the tibia from spinning, eliminating any spin out condition.